An adjuvant plays a role in promoting immune responses by accelerating or amplifying one or more specific phases of various immune responses. When an adjuvant is co-administered with an antigen, it can improve immunogenicity of the antigen and/or alter the type of immune response against the antigen. Typical examples of such adjuvants are an oil emulsion (Freund's adjuvant), monophosphoryl lipid A (MPL), Q saponins, aluminum hydroxide or phosphate or calcium salts (alum) of aluminum, non-ionic block polymer surfactants, lipopolysaccharides, mycobacteria, tetanus toxoid, CpG, etc.
Using a protein antigen alone does not often induce a sufficiently strong and a desired type of immune response, so vaccine compositions normally contain an antigen in combination with an adjuvant. According to a two signal model for the immune response, one signal delivered through the engagement of the antigen epitope presented with MHC molecule and antigen receptor is not enough to induce the immune response. It requires additional signals generated from co-stimulatory molecule(s). In this regard, the adjuvant may be able to enforce the signal strength generated by co-stimulatory molecules through induction of co-stimulatory molecules such as CD 40, CD 80, and CD 86 on antigen presenting cell (DC). It also induces MHC molecules and cytokines that determine the type of the immune response.
Some antigens such as lipoproteins, glycoproteins, or whole microorganisms can act both as an epitope and an adjuvant in the form of a pathogen associated molecular pattern (PAMP). The primary structure of a protein antigen, namely the amino acid sequence of an antigen, cannot be changed, but the PAMP of an antigen can be modified or supplemented by the addition of a proper adjuvant or subsidiary structure to affect immunogenicity (Dempsey P W et al., Science 271: 348-350, 1996; Deres K et al., Nature 342: 561-564, 1989).
The modification of a molecular pattern of an antigen can increase immunogenicity and also affect the type of elicited immune response. For example, in the case of an HBV surface antigen, S-protein without preS1 and preS2 does not exhibit immunogenicity in certain congenic mouse strains, while L-protein containing preS1 and preS2 not only induces the generation of antibody against preS1 and preS2 but also helps to induce the generation of antibody against S antigen (Milich D R et al., 1986, New Approaches to Immunization, pp 377-382. Cold Spring Harbor Laboratories, New York).
When a whole pathogenic microorganism is used as an antigen, it is expected that the microorganism contains various types of PAMP, such as lipopolysaccharides, nucleic acids, lipoproteins and conjugated proteins. In this case, the pathogen recognition receptor (PRR) existing on the surface of antigen presenting cell (APC) recognizes the PAMP to generate signals inducing various co-stimulatory molecules and cytokines, which affects the type of immune response as well as the level thereof. For example, interferon gamma and IL-12 helps to induce Th1 (T helper cell 1) response which plays an important role in immune response against virus infection. Th1 type immune response leads to the increase of IgG2a and IgG2b generation and induces a powerful cell mediated immune response. In this case, antigen associated various types of PAMPs act as an adjuvant and such adjuvant can help the regulation of immune responses.
A lipopeptide was first synthesized by Metzger et al. as a synthetic analogue of lipopeptide originated from bacteria and mycoplasma (Metzger J et al., Int J Peptide Protein Res 37:46-57, 1991). Since then, numerous analogues have been synthesized (EMC microcollections GmbH Sindelfinger Str. 3 72070 Tubingen, Germany). There is a report that virus-specific cytotoxic T lymphocyte (CTL) was induced by administrating a mouse with Pam3Cys-Ser-Ser, a lipopeptide conjugated with influenza virus T cell epitope (Schild H et al., Eur J Immunol 21:2649-2654, 1991). In general, the lipopeptide has been known as a TLR 2 ligand (Trinchieri G & Sher A, Nat Rev Immunol 7:179-190, 2007).
Poly I:C has been used as a powerful inducer of type I interferon in in vitro and in vivo studies (Magee M E & Griffith M J, life Science II, 11:1081-1086, 1972; Manetti Y R et al., Eur. J. Immunol. 25:2656-2660, 1995), and has been known to induce dendritic cell (DC) maturation, the most popular antigen presenting cell (APC) in mammals. The matured DC is capable of inducing immune response effectively (Rous R et al., International Immunol 16:767-773, 2004). Poly I:C is also known as an IL-12 inducer, and the IL-12 is an important cytokine, inducing cell mediated immune response and IgG2a antibody generation by promoting the enhancement of Th1 development. Adjuvant activity of poly I:C was also previously known (Cui Z & Qui F, Cancer Immunol Immunotherapy 16:1-13, 2005).
These types of natural adjuvants associated with the antigens, however, often may not be strong enough to induce a desired strength and a quality of immune response, requiring a good adjuvant in a vaccine formulation.
Developing a good adjuvant is accordingly a very important job in developing a good vaccine, but adjuvant development still has to rely mainly on empirical work. For example, Toll Like Receptors (TLR) are the most important PRR on antigen presenting cells (APC) involved in the activation of APC and in antigen presentation by APC. Potent antibody response, however, is not entirely dependent on TLR signals (Gavin. A. L. et al, Science 314:1936-1938, 2006). Further, Pam3cys, which is a TLR2 ligand, works in inducing immune response independently of TLR2 (Yoder et al, Infect. Immun. 71:3894-3900, 2003). Accordingly, although Pam3Cys and poly I:C are known to be synergistic in inducing TNF-α and IL-6 in macrophages (Bagchi et al, J. Immun. 178:1164-1171, 2007), no teaching of a well balanced powerful adjuvant function of a similar combination consisting of a lipopeptide and poly I:C is found in the prior art. To make matters further complicated, good protective immune response requires balanced immune response comprising both strong cell mediated immune response and humoral antibody response. Therefore, developing an adjuvant that will help to induce well balanced adaptive immune response still can not be rationally predicted.